1. Technical Field
The present invention relates to a method of controlling a liquid ejecting apparatus, such as an ink jet printer, and to a liquid ejecting apparatus. More particularly, the invention relates to a method of driving a liquid ejecting head that ejects liquid drops from a nozzle opening by operating a pressure-generating element as a result of supplying a driving signal; and to a liquid ejecting apparatus.
2. Related Art
A liquid ejecting apparatus includes a liquid ejecting head that can eject a liquid as drops of liquid, and ejects various types of liquids from the liquid ejecting head. A typical example of the liquid ejecting apparatus is an image recording apparatus, such as an ink jet printer (hereafter simply referred to as “printer”). The printer performs a recording operation as a result of forming dots by ejecting liquid ink as drops of ink onto, for example, a recording sheet (serving as an ejection object onto which ejection is performed) and by causing the ink drops to land onto the recording sheet. In recent years, a liquid ejecting apparatus is applied not only to the image recording apparatus, but also to various types of manufacturing apparatuses, such as a display manufacturing apparatus.
Here, the aforementioned printer is taken as an example. It includes a recording head and a driving-signal generating circuit (driving vibration generating unit). The recording head includes, for example, ink paths and a pressure-generating element (such as a piezoelectric element). The ink paths extend to openings of nozzles extending through a pressure-generating chamber from a common ink chamber (reservoir). The pressure-generating element varies the volume of the pressure-generating chamber. The driving-signal generating circuit generates driving signals that are supplied to the piezoelectric element. The piezoelectric element is driven on the basis of driving pulses, included in the driving signals, from the driving-signal generating circuit, to vary the pressure of ink in the pressure-generating chamber. Then, the variation in pressure is made use of to eject ink drops from the nozzle openings.
In this type of printer, a demand for higher quality of a recording image is causing the ink drops to be ejected to become very small. That is, the diameter of dots that are recorded onto a recording medium, such as a recording sheets is reduced as a result of making the ink drops very small, to achieve higher resolution of the recording image and to reduce the roughness of the image that a person perceives visually in a low-density area. The ink drops may be made very small as a result of reducing the diameter of the nozzle openings. However, when the diameter of the nozzle openings is reduced, processing becomes difficult, thereby tending to reduce precision in addition to increasing costs. In addition, clogging tends to occur as a result of drying of the ink near the nozzle openings, thereby placing a limit on how small the diameter of the nozzle openings can be reduced.
Therefore, a technology which makes ink drops very small without changing the size of the nozzle openings has been proposed. In the technology, this is achieved by controlling a meniscus behavior during the ejection of ink drops by putting some thought in forming a driving signal for driving a piezoelectric element. For example, JP-A-2002-127418 (FIGS. 3 and 4) discloses the following ink jet recording apparatus. In the apparatus, a driving signal is provided with a contraction signal used to temporarily contract a pressure-generating chamber prior to providing a preparation signal used to draw in a meniscus as a result of expanding the pressure-generating chamber before ejecting ink drops. The meniscus is pushed out on the basis of the contraction signal. Then, the subsequent preparation signal is used to locally draw in a portion near the center of the meniscus, so that the ink of very small portions near the center of the drawn-in meniscus are discharged as very small ink drops.
However, when the ink drops are made very small without taking any measures, a fly speed during ejection is reduced. This may cause, for example, bending of the flying, or formation of mists as a result of the ink drops not being able to land onto an ejection object (such as a recording sheet).
Residual vibration of ink becomes a problem after the ejection of ink. That is, the residual vibration causes the meniscus to behave improperly. Therefore, ink drops may be accidentally ejected, or the next ejection of ink drops may be adversely affected. In particular, when very small ink drops are successively ejected in a very short time (such as a few μs), it is desirable to restrict the residual vibration to the extent possible.